1. Field of the Invention
The present invention relates to a wireless access method, device and system, and more particularly, to a method for performing zone switch based handover.
2. Discussion of the Background Art
Handover refers to movement of a mobile station (MS) from a wireless interface of one base station (BS) to a wireless interface of another BS. Hereinafter, a handover procedure in a general IEEE 802.16e system will be described.
A serving BS in an IEEE 802.16e network may broadcast information about neighbor BSs through a mobile neighbor advertisement (MOB_NBR-ADV) message in order to inform an MS of information (topology) about basic network configuration.
The MOB_NBR-ADV message includes system information about a serving BS and neighbor BSs, for example, a preamble index, a frequency, a handover optimization capability, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information of which an MS should be aware in order to exchange information through downlink and uplink. For example, the DCD/UCD information may include handover trigger information, and medium access control (MAC) version and media independent handover (MIH) capability information of a BS.
A general MOB_NBR-ADV message includes information about neighbor BSs of an IEEE 802.16e type only. Accordingly, information about neighbor BSs having a type other than the IEEE 802.16e type may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. As a result, an MS may acquire information about BSs of a heterogeneous network by requesting a serving BS to transmit the SII-ADV message.
A procedure for an MS having information about neighbor BSs obtained through the above-described method to perform handover in an IEEE 802.16e network will be described in more detail.
A handover procedure in a general IEEE 802.16e network may be classified into three processes: handover initiation and preparation, handover execution, and handover completion.
An example of a basic handover procedure which may be constructed as described above will now be described with reference to FIG. 1.
FIG. 1 illustrates an example of a handover procedure which can be performed in an IEEE 802.16e system.
Referring to FIG. 1, an MS exchanges data with a serving BS (SBS) (step S101).
The SBS periodically broadcasts information about neighbor BSs to the MS through a MOB_NBR-ADV message (step S102).
The MS may start scanning for candidate handover BSs using a handover (HO) trigger condition while communicating with the SBS. The MS requests the SBS to perform a handover procedure by transmitting a handover request (MOB_MSHO-REQ) message when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded (step S103).
The SBS informs candidate handover BSs included in the MOB_MSHO-REQ message that the MS has requested handover through a handover request (HO-REQ) message (step S104).
The candidate handover BSs take action for the MS having requested handover to transmit information about handover to the SBS through a handover response (HO-RSP) message (step S105).
The SBS transmits the information about handover, obtained through the HO-RSP message from the candidate handover BSs, to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information necessary to perform handover, that is, a handover action time, a handover identifier (ID), and a dedicated handover code division multiple access (CDMA) ranging code (step S106).
The MS determines one target BS (TBS) among the candidate BSs based on the information included in the MOB-BSHO-RSP message received from the SBS. The MS then transmits a CDMA code to the determined TBS to attempt ranging (step S107).
The TBS receiving the CDMA code may inform the MS of success or failure of ranging and physical correction values through a ranging response (RNG-RSP) message (step S108).
The MS transmits a ranging request (RNG-REQ) message for authentication to the TBS (step S109).
The TBS receiving the RNG-REQ message from the MS transmits system information, which can be used in a corresponding BS, such as a connection identifier (CID) to the MS through a ranging response (RNG-RSP) message (step S110).
If the TBS successfully completes authentication of the MS and transmits all update information, the TBS informs the SBS of success or failure of handover through a handover completion (HO-CMPT) message (step S111).
Next, the MS exchanges data with the TBS which has performed handover (step S112).
It is assumed that the above-described handover process is performed between an MS and a BS conforming to the IEEE 802.16e specification (wirelessMAN-OFDMA reference system). Hereinafter, a system to which a general technique including the IEEE 802.16e specification is applied will be referred to as a ‘legacy system’. An MS to which a legacy technique is applied will be referred to as a ‘yardstick MS (YMS)’ or a ‘legacy MS’, and a BS to which a legacy technique is applied will be referred to as a ‘legacy BS’ or ‘yardstick BS (YBS)’.
Meanwhile, an MS and a BS to which a more advanced technique, including the IEEE 802.16m specification (wirelessMAN-OFDMA advanced system), than a general technique is applied will be referred to as an ‘advanced MS (AMS)’ and an ‘advanced BS (ABS)’, respectively.
It is assumed that an AMS is connected to a YBS which provides a service thereto and an ABS (wirelessMAN-OFDMA reference system/wirelessMAN-OFDMA advanced co-existing system) which supports both the AMS and the YMS is adjacent to the YBS.
It is assumed that the YBS includes only a legacy zone (LZone) having a physical channel frame structure applied to a legacy system, and the ABS includes only an AMS support zone (MZone or 16 m zone) having a physical channel frame structure applied to an advanced system when an ABS (wirelessMAN-OFDMA advanced system) supports only the AMS. It is assumed that an ABS (wirelessMAN-OFDMA reference system/wirelessMAN-OFDMA advanced co-existing system) which supports both the AMS and the YMS includes the LZone and the MZone and is divided by time division duplex (TDD) on a time basis, for example, on a frame or subframe basis in each of uplink and downlink.
It is also assumed that a service can be provided to the AMS by both the ABS and the YBS. Namely, it is assumed that a service can be provided to the AMS through any one of the MZone and the LZone and the AMS can perform a handover execution process defined both in the legacy system and in the advanced system.
Generally, in order to perform handover from a serving YBS to an ABS which supports both the AMS and the YMS, the AMS first enters an LZone of the ABS, and then continues to be served in the LZone or performs zone switching to an MZone of the ABS. Alternatively, the AMS may perform handover by immediately performing zone switching to the MZone without entering the LZone of the ABS.